1. Technical Field
The present invention relates to a hydraulic shock absorber which is capable of expelling air bubbles collected inside an oil chamber, into an air chamber.
2. Related Art
In a front fork of a motorcycle which is one example of a hydraulic shock absorber, bushes are fixed to the top and bottom of an inner circumference of an outer tube, an inner tube is fitted slidably via these upper and lower bushes, and a ring-shaped oil chamber defined by the upper and lower bushes is formed in the space between the inner circumference of the outer tube and the outer circumference of the inner tube, whereby a volume compensating chamber for the entry and withdrawal of a piston rod during a stroke is formed. In a front fork of this kind, a partition wall member is installed on the inside of an opening end side of the inner tube, in order to guide the piston rod which is installed on the outer tube. The partition wall member is constituted by a cylinder section which is formed in a cylindrical shape along the inner circumference of the inner tube, and a bottom section which is provided so as to close off the bottom of the cylindrical section, and by screw coupling the cylinder section with the inner tube, the bottom section is positioned inside the inner tube and the bottom section forms a reference which defines a hydraulic oil chamber in which a piston attached to the piston rod fixed to an outer tube performs a sliding motion, below the bottom section, and an oil reservoir chamber for supplying and expelling hydraulic oil to and from the hydraulic oil chamber, above the bottom section. This hydraulic oil chamber is connected to the ring-shaped oil chamber via an oil hole which is provided in the inner tube, whereby the hydraulic oil can pass freely, and when the front fork is compressed, hydraulic oil of an amount corresponding to the volume of the piston rod that has entered into the hydraulic oil chamber during the stroke flows out from the hydraulic oil chamber and into the ring-shaped oil chamber, and when the front fork is extended, hydraulic oil of an amount corresponding to the volume of the piston rod that has withdrawn during the stroke from the hydraulic oil chamber flows into the hydraulic oil chamber from the ring-shaped oil chamber, whereby volume compensation for entry and withdrawal of the piston rod is performed and a prescribed damping force is obtained (Patent Literature 1 (JP-A-2009-108884)).
However, in a hydraulic shock absorber having a composition of this kind, since air bubbles contained in the hydraulic oil during operation gradually collect in the ring-shaped oil chamber and a gap between the partition wall member and the inner tube, then a collection of unwanted air bubbles arises, and hence the prescribed damping force is obtained after this collection of air bubbles has been compressed by the hydraulic oil during a compression operation, and a problem arises in that slackness occurs in the damping action. Therefore, in order to resolve problems of this kind, Patent Literature 2 (JP-A-2010-236577) discloses a composition in which an air escape path connecting a ring-shaped oil chamber and an oil reservoir chamber is provided, and this air escape path is opened and closed by an O-ring that functions as a check valve performing an opening and closing operation in accordance with an extension and compression action of the air escape path, and the collection of air bubbles present in the ring-shaped oil chamber is pushed out into the oil reservoir chamber during an external operation.
However, in the hydraulic shock absorber relating to the composition of this kind, since the check valve, such as the O-ring, is required, then this composition is difficult to adopt from the viewpoint of reducing space requirements, reducing the number of components, and restricting manufacturing costs by simplifying the structure.